The present invention relates to a watertight and thermally insulating tank, particularly for storing a liquefied gas, such as methane, at a temperature of about xe2x88x92160xc2x0 C., said tank being built into the bearing structure of a ship. The invention also relates to a method of manufacturing thermally insulating caissons intended to be used in this tank.
French patent No 2 527 544 belonging to the current applicant, discloses a watertight and thermally insulating tank built into the bearing structure of a ship, said tank comprising two successive watertightness barriers, one of them a primary one in contact with the product contained in the tank and the other a secondary one located between the primary barrier and the bearing structure, these two watertightness barriers alternating with two thermally insulating barriers known as the primary and secondary insulating barriers each thermally insulating barrier consisting of a number of caissons of roughly parallelepipedal overall shape, each caisson comprising a bottom panel and a top panel made of plywood, side walls and internal partitions, each caisson being filled with a thermally insulating particulate material, for example perlite. However, the use of a powder such as perlite complicates the manufacture of the caissons, because the powder produces dust and it is necessary to use a high-quality and therefore expensive plywood in order to correctly seal the caisson against dust, that is to say to use a plywood which has no knots, and it is necessary to compact the powder at a given pressure in the caisson and it is necessary to pass nitrogen through each caisson to remove all the air present, for safety reasons. All of these operations make manufacture more complicated and increase the cost of the caissons.
It is also known practice, see French patent No 2 724 623, to use insulating caissons consisting of two plywood panels between which is bonded a thermally insulating layer of cellular plastic, such as a polyurethane foam, possibly reinforced with fiberglass fabric inserted into said foam to give it good mechanical properties. The use of a foam avoids the problem associated with the particulate nature of the perlite, and therefore makes it possible to use a lower grade of plywood. By contrast, it is necessary to use a high-density foam, for example one with a density of the order of 120 kg/m3, to guarantee mechanical support of the watertightness barriers subjected to the pressure and movements of the cargo. The fact of using a high-density foam increases its cost and reduces its insulation ability. Thus, it is necessary to increase the thickness of the insulating barrier, which leads to a reduction in the interior tank volume.
The object of the present invention is therefore to propose a tank, the insulating caissons of which do not exhibit the aforementioned drawbacks but which, on the contrary, display good thermal insulation while at the same time being of a simple structure and low cost.
To this end, the subject of the invention is a watertight and thermally insulating tank built into the bearing structure of a ship, said tank comprising two successive watertightness barriers, one being a primary one in contact with the product contained in the tank and the other being a secondary one placed between the primary barrier and the bearing structure, at least one thermally insulating barrier being provided between the bearing structure and the secondary watertightness barrier and/or between the two watertightness barriers, each thermally insulating barrier consisting of a number of caissons of roughly parallelepipedal overall shape, each caisson having a bottom panel and a top panel made of plywood, characterized in that the panels of each caisson are spaced apart by a number of spacer pieces which consist of thin sheets of plywood, for example of the order of 4 mm thick, extending at right angles to said panels, each caisson being at least partially filled with blocks of foam, preferably a low-density foam with a density of the order of 33 to 40 kg/m3, bonded over a substantial part of the height of each spacer piece, to prevent the spacer pieces from buckling under load. By using a low-density foam, the thermal insulation is better, which makes it possible to reduce the thickness of the insulating barrier and therefore increase the interior tank volume. Furthermore, by using spacer pieces of the order of 4 mm thick, instead of the 9 mm internal partitions of the caissons of the prior art, it is possible to reduce the cost of manufacture of the caisson.
By way of example, the parameter xcex representing the insulation of an insulating caisson is of the order of 0.043 kcal per meter of caisson thickness, per xc2x0 C. and per m2 of caisson surface area, for perlite, whereas this parameter xcex is of the order of 0.030 for a caisson made of a layer of high-density foam sandwiched between two plywood panels, and is of the order of 0.011 to 0.015 for the caisson of the invention. It can thus be seen that, for same thickness, the caisson of the invention is a far better insulator because the heat transfer is lower.
Advantageously, each intermediate space between two spacer pieces of a caisson contains at least one block of foam which is bonded to the walls facing each other on said spacer pieces and extends from one wall to the other.
As a preference, the blocks of foam completely fill the caisson.
According to another feature, the spacer pieces constitute mutually parallel internal partitions of the caisson fixed to said panels at regular intervals. In this case, it is possible to make provision for said partitions to extend over the entire length of the caisson and for the two outermost lateral partitions of the caisson to be spaced from the free edges of the caisson by a half interval filled with foam.
According to yet another feature, the watertightness barriers consist of metal strakes with edges turned up toward the inside of the tank, said strakes being made of thin plate with a low coefficient of expansion and being butt-welded, via their turned-up edges, onto the two faces of a weld flange, for example in the shape of an angle bracket, which is held mechanically on the caissons of the insulating barriers by an expansion joint, said weld flanges being partially engaged in parallel slots, for example T-shaped slots, formed in the top panel of the caissons, the distance between two slots corresponding to the width of a strake, whereas the distance between the free edge of a caisson and the adjacent slot corresponds to the width of half a strake, so that another strake the same width joins two adjacent caissons together.
In this case, it is possible to make provision for the caissons of the primary insulating barrier to have slits through their bottom walls to accommodate, with sliding, the weld flanges of the secondary watertightness barrier, said slits being perpendicular to the spacer pieces of the primary insulating caissons.
In one particular embodiment, each caisson has, at its four corners, a well which passes through the top panel and the blocks of foam, one wall of the well corresponding to one wall of the outermost lateral spacer piece and the bottom of the well consisting of the bottom panel of the caisson, so that the bottom of the well supports laths intended to collaborate with members for fixing the caissons to the bearing structure, the bottom of each well having a recess through the bottom panel for the passage of said fixing members. In this case, the caissons of the insulating barriers are arranged side by side contiguously without any gaps in between, the laths being housed in the wells at each corner of the caissons, without projecting beyond the lateral walls of the caisson.
Advantageously, the members for fixing the secondary insulating barrier to the bearing structure consist of rods, the base of which is screwed into a socket welded to the bearing structure of the ship, said rods passing through the recesses made in the corners of four adjacent secondary caissons, each rod being equipped at its upper part with a mount plate, preferably made of metal, resting against four adjacent laths of four adjacent secondary caissons, placed around said rod, the clamping of the mount plate onto the laths being achieved using a nut which can be screwed onto the threaded upper end of said rod, at least one Belleville washer preferably being inserted between the nut and said mount plate.
In this case, it is possible to make provision for a piece of plywood to be inserted between said mount plate and another mount plate so that said other mount plate lies exactly flush with the level of the top panel of the caissons of the secondary insulating barrier, the two mount plates and the piece of plywood being joined together by screws, the upper mount plate having, at its center, a threaded bore for the fixing of the members for fixing the primary insulating barrier.
As a preference, the strakes of the secondary watertightness barrier which rest against the caissons of the secondary insulating barrier are pierced, in line with said threaded bores so that a threaded base of a connector which has a peripheral rim resting against said strake can be screwed into them, this rim being welded continuously to the strake to restore the watertightness of the secondary watertightness barrier, this rim being extended by another threaded rod, the upper end of which is fitted with a nut for clamping a mount plate against the four adjacent laths of four adjacent caissons of the primary insulating barrier, preferably via at least one Belleville washer.
The invention is also aimed at a method of manufacturing thermally insulating caissons intended to be used in the tank defined hereinabove, characterized in that it consists in stacking, alternately, a number of layers of foam, preferably low density foam, and a number of sheets of plywood, inserting adhesive between each layer of foam and each sheet, until the height of said stack corresponds to the length of said caissons, in cutting the aforementioned stack in slices in the heightwise direction at regular intervals that correspond to the thickness of a caisson, and in bonding onto the edge faces of each slice of stack thus cut, on one side, a bottom panel and on the other side, a top panel made of plywood, said panels extending at right angles to said cut sheets which act as spacer pieces.